Map by Matthew Sibol, using data from a U. S. Geological Survey Earthquake Catalog
COUNTING only those earthquakes that register a magnitude of 2.0 or greater on the Richter scale, there are more than two quakes a day, on average, in the lower forty-eight states. Although it takes a quake of 5.5 or greater to cause significant damage to buildings, a 2.0 quake will still give a rude jolt to people within a few miles of its epicenter.
The states along the Pacific coast, of course, have the highest concentration of quakes, because the fault lines that run through this region constitute a boundary between two giant tectonic plates: the North American plate and the Pacific plate. Four of the five largest quakes during the twenty-year period covered by this map were in California or off its coast.
But seismic activity is also common in places far away from this plate boundary. A prominent band of earthquakes extends northward from Nevada to Montana, in a zone between deformed rocks and stable rocks which has been active periodically for the past 600 million years. Another band of earthquake activity extends westward from Yellowstone National Park. The cause of these earthquakes, and of the unusual volcanic activity at Yellowstone, is the presence of a "hot spot"--a pipe of molten rock that blasts its way up to the surface. (Pressure from this molten rock was so intense from 1923 to 1984 that it gradually raised the park's elevation by three feet.)